Bioconversion of renewable lignocellulosic biomass to a fermentable sugar that is subsequently fermented to produce an alcohol (e.g., ethanol or “bioethanol”), which can serve as an alternative to liquid fuels, has attracted intensive attention of researchers since the 1970s, when the oil crisis occurred because OPEC decreased the output of petroleum (Bungay, “Energy: the biomass options”. NY: Wiley; 1981; Olsson and Hahn-Hagerdal, 1996, Enzyme Microb. Technol. 18:312-31; Zaldivar et al., 2001, Appl. Microbiol. Biotechnol. 56:17-34; Galbe and Zacchi, 2002, Appl. Microbiol. Biotechnol. 59:618-28). Ethanol has been widely used as a 10% blend to gasoline in the USA or as a neat fuel for vehicles in Brazil in the last two decades. The importance of fuel bioethanol will increase in parallel with skyrocketing prices for oil and gradual depletion of its sources.
Lignocellulosic biomass is predicted to be a low-cost renewable resource that can support the sustainable production of biofuels (e.g., bioethanol) on a large enough scale to significantly address the world's increasing energy needs. Lignocellulosic materials include, without limitation, corn stover (the corn plant minus the kernels and the roots), forestry residues such as sawdust and paper, yard waste from municipal solid waste, herbaceous plants such as switchgrass, and woody plants such as poplar trees. Lignocellulosic biomass has three major components: hemicellulose, cellulose, and lignin. Hemicellulose is an amorphous, branched polymer that is usually composed primarily of five sugars (arabinose, galactose, glucose, mannose, and xylose). Cellulose is a large, linear polymer of glucose molecules typically joined together in a highly crystalline structure due to hydrogen bonding between parallel chains. Lignin is a complex phenyl-propane polymer.
The biological processing of lignocellulosic biomass involves using cellulases and hemicellulases to release sugars from hemicellulose and cellulose, respectively, typically by hydrolysis reactions. The resulting sugars are then fermented into biofuels such as bioethanol using suitable fermenting microorganisms.
The glucose released when cellulose is broken down by cellulases can often be a potent inhibitor of this class of enzymes. To reduce glucose accumulation during cellulose breakdown (or “saccharification” herein), a fermenting microorganism can be added to convert the released sugars into bioethanol at the same time the sugars are revealed from saccharification. This configuration is called simultaneous saccharification and fermentation (“SSF”). Generally, SSF offers better/higher rates, yields, and concentrations of ethanol produced than a separate hydrolysis and fermentation (“SHF”) configuration, despite operating at lower temperatures than are optimal for most enzymes involved in these fermentation processes. Nonetheless, the typical SSF reaction can be exceedingly lengthy, lasting, for example, several days in order to achieve modest ethanol concentrations (see, e.g., Kadam et al., 2004, Biotechnol. Progr. 20(3):705).
Accordingly, there exists a need in the art to identify methods and compositions related thereto for improving the efficiency of SSF reactions and increasing the yield of biofuels such as bioethanol.